The invention relates to a method and a device for obtaining an electronic half pixel offset, and to a camera including such a device.
European Patent Application EP-A-0,476,421 discloses a circuit for producing a brightness signal from an output signal of a solid-state image pick-up apparatus using spatial pixel shift. In a color camera, red, green and blue solid-state image sensors each have a number of light-receiving elements arranged in a matrix to derive red, green and blue color signals. The light-receiving elements of the green solid-state image sensor are shifted in the horizontal scanning direction with respect to those of the red and blue solid-state image sensors over a distance which is equal to a half of a pitch of the arrangement of the light-receiving elements in the horizontal scanning direction. The camera further comprises a delay circuit for delaying the sampled green color signal by a delay time which corresponds to this half pitch.
The presence of delay lines constitutes a problem in the case of scan reversal, as the electronic correction in the green channel then has to change its sign.
It is, inter alia, an object of the invention to provide an electronic half pixel offset without using delay lines to compensate for the half pixel offset of the green sensor with regard to the blue and red sensors. To this end, a first aspect of the invention provides a method and a device for obtaining an electronic half pixel offset. A second aspect of the invention provides a method and a camera for generating red, green and blue color signals.
In a method of generating red, green and blue color signals in accordance with a primary aspect of the invention, the red, green and blue color signals are picked up by means of red, green and blue color sensors each having a horizontal read-out register, while while first n-phase clock signals, generated from a pixel clock signal, are processed to obtain second n-phase clock signals for the read-out register of the green color sensor in order to obtain an electronic half pixel offset for the green color signal with respect to the red and blue color signals. A correction delay is thereby avoided. Delay correction at scan reversal is also simplified: one only needs to shift the starting instant of the pulses of the horizontal shift register over one pixel clock pulse. The invention is cheaper than the prior art solution and can more easily be cast in an integrated circuit. Moreover, the half pixel offset can be made in a more accurate manner.
In a preferred embodiment, the green signal is shifted in time over exactly half a pixel period by inverting the phase of the clock pulses for the green sensor with regard to the phases of the clock pulses for the red and blue sensors during shifting of the green color signal out of the horizontal shift register of the green sensor. This holds for 2 and 3-phase horizontal registers. For a 4-phase horizontal register, the pulses need to be interchanged cyclically.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.